docs/libcurl/libcurl-multi.3 - third_party/curl - Git at Google

 .\" **************************************************************************
 .\" *                                  _   _ ____  _
 .\" *  Project                     ___| | | |  _ \| |
 .\" *                             / __| | | | |_) | |
 .\" *                            | (__| |_| |  _ <| |___
 .\" *                             \___|\___/|_| \_\_____|
 .\" *
 .\" * Copyright (C) 1998 - 2019, Daniel Stenberg, <daniel@haxx.se>, et al.
 .\" *
 .\" * This software is licensed as described in the file COPYING, which
 .\" * you should have received as part of this distribution. The terms
 .\" * are also available at https://curl.haxx.se/docs/copyright.html.
 .\" *
 .\" * You may opt to use, copy, modify, merge, publish, distribute and/or sell
 .\" * copies of the Software, and permit persons to whom the Software is
 .\" * furnished to do so, under the terms of the COPYING file.
 .\" *
 .\" * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
 .\" * KIND, either express or implied.
 .\" *
 .\" **************************************************************************
 .\"
 .TH libcurl-multi 3 "19 Sep 2014" "libcurl" "libcurl multi interface"
 .SH NAME
 libcurl-multi \- how to use the multi interface
 .SH DESCRIPTION
 This is an overview on how to use the libcurl multi interface in your C
 programs. There are specific man pages for each function mentioned in
 here. There's also the \fIlibcurl-tutorial(3)\fP man page for a complete
 tutorial to programming with libcurl and the \fIlibcurl-easy(3)\fP man page
 for an overview of the libcurl easy interface.

 All functions in the multi interface are prefixed with curl_multi.
 .SH "OBJECTIVES"
 The multi interface offers several abilities that the easy interface doesn't.
 They are mainly:

 1. Enable a "pull" interface. The application that uses libcurl decides where
 and when to ask libcurl to get/send data.

 2. Enable multiple simultaneous transfers in the same thread without making it
 complicated for the application.

 3. Enable the application to wait for action on its own file descriptors and
 curl's file descriptors simultaneously.

 4. Enable event-based handling and scaling transfers up to and beyond
 thousands of parallel connections.
 .SH "ONE MULTI HANDLE MANY EASY HANDLES"
 To use the multi interface, you must first create a 'multi handle' with
 \fIcurl_multi_init(3)\fP. This handle is then used as input to all further
 curl_multi_* functions.

 With a multi handle and the multi interface you can do several simultaneous
 transfers in parallel. Each single transfer is built up around an easy
 handle. You create all the easy handles you need, and setup the appropriate
 options for each easy handle using \fIcurl_easy_setopt(3)\fP.

 There are two flavours of the multi interface, the select() oriented one and
 the event based one we call multi_socket. You will benefit from reading
 through the description of both versions to fully understand how they work and
 differentiate. We start out with the select() oriented version.

 When an easy handle is setup and ready for transfer, then instead of using
 \fIcurl_easy_perform(3)\fP like when using the easy interface for transfers,
 you should add the easy handle to the multi handle with
 \fIcurl_multi_add_handle(3)\fP. You can add more easy handles to a multi
 handle at any point, even if other transfers are already running.

 Should you change your mind, the easy handle is again removed from the multi
 stack using \fIcurl_multi_remove_handle(3)\fP. Once removed from the multi
 handle, you can again use other easy interface functions like
 \fIcurl_easy_perform(3)\fP on the handle or whatever you think is
 necessary. You can remove handles at any point in time during transfers.

 Adding the easy handle to the multi handle does not start the transfer.
 Remember that one of the main ideas with this interface is to let your
 application drive. You drive the transfers by invoking
 \fIcurl_multi_perform(3)\fP. libcurl will then transfer data if there is
 anything available to transfer. It'll use the callbacks and everything else
 you have setup in the individual easy handles. It'll transfer data on all
 current transfers in the multi stack that are ready to transfer anything. It
 may be all, it may be none. When there's nothing more to do for now, it
 returns back to the calling application.

 Your application extracts info from libcurl about when it would like to get
 invoked to transfer data or do other work. The most convenient way is to use
 \fIcurl_multi_poll(3)\fP that will help you wait until the application should
 call libcurl again. The older API to accomplish the same thing is
 \fIcurl_multi_fdset(3)\fP that extracts fd_sets from libcurl to use in
 select() or poll() calls in order to get to know when the transfers in the
 multi stack might need attention. Both these APIs allow for your program to
 wait for input on your own private file descriptors at the same time
 \fIcurl_multi_timeout(3)\fP also helps you with providing a suitable timeout
 period for your select() calls.

 \fIcurl_multi_perform(3)\fP stores the number of still running transfers in
 one of its input arguments, and by reading that you can figure out when all
 the transfers in the multi handles are done. 'done' does not mean
 successful. One or more of the transfers may have failed.

 To get information about completed transfers, to figure out success or not and
 similar, \fIcurl_multi_info_read(3)\fP should be called. It can return a
 message about a current or previous transfer. Repeated invokes of the function
 get more messages until the message queue is empty. The information you
 receive there includes an easy handle pointer which you may use to identify
 which easy handle the information regards.

 When a single transfer is completed, the easy handle is still left added to
 the multi stack. You need to first remove the easy handle with
 \fIcurl_multi_remove_handle(3)\fP and then close it with
 \fIcurl_easy_cleanup(3)\fP, or possibly set new options to it and add it again
 with \fIcurl_multi_add_handle(3)\fP to start another transfer.

 When all transfers in the multi stack are done, close the multi handle with
 \fIcurl_multi_cleanup(3)\fP. Be careful and please note that you \fBMUST\fP
 invoke separate \fIcurl_easy_cleanup(3)\fP calls for every single easy handle
 to clean them up properly.

 If you want to re-use an easy handle that was added to the multi handle for
 transfer, you must first remove it from the multi stack and then re-add it
 again (possibly after having altered some options at your own choice).
 .SH "MULTI_SOCKET"
 \fIcurl_multi_socket_action(3)\fP function offers a way for applications to
 not only avoid being forced to use select(), but it also offers a much more
 high-performance API that will make a significant difference for applications
 using large numbers of simultaneous connections.

 \fIcurl_multi_socket_action(3)\fP is then used instead of
 \fIcurl_multi_perform(3)\fP.

 When using this API, you add easy handles to the multi handle just as with the
 normal multi interface. Then you also set two callbacks with the
 \fICURLMOPT_SOCKETFUNCTION(3)\fP and \fICURLMOPT_TIMERFUNCTION(3)\fP options
 to \fIcurl_multi_setopt(3)\fP. They are two callback functions that libcurl
 will call with information about what sockets to wait for, and for what
 activity, and what the current timeout time is - if that expires libcurl
 should be notified.

 The multi_socket API is designed to inform your application about which
 sockets libcurl is currently using and for what activities (read and/or write)
 on those sockets your application is expected to wait for.

 Your application must make sure to receive all sockets informed about in the
 \fICURLMOPT_SOCKETFUNCTION(3)\fP callback and make sure it reacts on the given
 activity on them. When a socket has the given activity, you call
 \fIcurl_multi_socket_action(3)\fP specifying which socket and action there
 are.

 The \fICURLMOPT_TIMERFUNCTION(3)\fP callback is called to set a timeout. When
 that timeout expires, your application should call the
 \fIcurl_multi_socket_action(3)\fP function saying it was due to a timeout.

 This API is typically used with an event-driven underlying functionality (like
 libevent, libev, kqueue, epoll or similar) with which the application
 "subscribes" on socket changes. This allows applications and libcurl to much
 better scale upward and beyond thousands of simultaneous transfers without
 losing performance.

 When you've added your initial set of handles, you call
 \fIcurl_multi_socket_action(3)\fP with CURL_SOCKET_TIMEOUT set in the sockfd
 argument, and you'll get callbacks call that sets you up and you then continue
 to call \fIcurl_multi_socket_action(3)\fP accordingly when you get activity on
 the sockets you've been asked to wait on, or if the timeout timer expires.

 You can poll \fIcurl_multi_info_read(3)\fP to see if any transfer has
 completed, as it then has a message saying so.
 .SH "BLOCKING"
 A few areas in the code are still using blocking code, even when used from the
 multi interface. While we certainly want and intend for these to get fixed in
 the future, you should be aware of the following current restrictions:

 .nf
  - Name resolves unless the c-ares or threaded-resolver backends are used
  - SOCKS proxy handshakes
  - file:// transfers
  - TELNET transfers
 .fi
 .SH "SEE ALSO"
 .BR libcurl-errors "(3), " libcurl-easy "(3), " libcurl "(3) "
	.\" **************************************************************************
	.\" * _ _ ____ _
	.\" * Project ___\| \| \| \| _ \\| \|
	.\" * / __\| \| \| \| \|_) \| \|
	.\" * \| (__\| \|_\| \| _ <\| \|___
	.\" * \___\|\___/\|_\| \_\_____\|
	.\" *
	.\" * Copyright (C) 1998 - 2019, Daniel Stenberg, <daniel@haxx.se>, et al.
	.\" *
	.\" * This software is licensed as described in the file COPYING, which
	.\" * you should have received as part of this distribution. The terms
	.\" * are also available at https://curl.haxx.se/docs/copyright.html.
	.\" *
	.\" * You may opt to use, copy, modify, merge, publish, distribute and/or sell
	.\" * copies of the Software, and permit persons to whom the Software is
	.\" * furnished to do so, under the terms of the COPYING file.
	.\" *
	.\" * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
	.\" * KIND, either express or implied.
	.\" *
	.\" **************************************************************************
	.\"
	.TH libcurl-multi 3 "19 Sep 2014" "libcurl" "libcurl multi interface"
	.SH NAME
	libcurl-multi \- how to use the multi interface
	.SH DESCRIPTION
	This is an overview on how to use the libcurl multi interface in your C
	programs. There are specific man pages for each function mentioned in
	here. There's also the \fIlibcurl-tutorial(3)\fP man page for a complete
	tutorial to programming with libcurl and the \fIlibcurl-easy(3)\fP man page
	for an overview of the libcurl easy interface.

	All functions in the multi interface are prefixed with curl_multi.
	.SH "OBJECTIVES"
	The multi interface offers several abilities that the easy interface doesn't.
	They are mainly:

	1. Enable a "pull" interface. The application that uses libcurl decides where
	and when to ask libcurl to get/send data.

	2. Enable multiple simultaneous transfers in the same thread without making it
	complicated for the application.

	3. Enable the application to wait for action on its own file descriptors and
	curl's file descriptors simultaneously.

	4. Enable event-based handling and scaling transfers up to and beyond
	thousands of parallel connections.
	.SH "ONE MULTI HANDLE MANY EASY HANDLES"
	To use the multi interface, you must first create a 'multi handle' with
	\fIcurl_multi_init(3)\fP. This handle is then used as input to all further
	curl_multi_* functions.

	With a multi handle and the multi interface you can do several simultaneous
	transfers in parallel. Each single transfer is built up around an easy
	handle. You create all the easy handles you need, and setup the appropriate
	options for each easy handle using \fIcurl_easy_setopt(3)\fP.

	There are two flavours of the multi interface, the select() oriented one and
	the event based one we call multi_socket. You will benefit from reading
	through the description of both versions to fully understand how they work and
	differentiate. We start out with the select() oriented version.

	When an easy handle is setup and ready for transfer, then instead of using
	\fIcurl_easy_perform(3)\fP like when using the easy interface for transfers,
	you should add the easy handle to the multi handle with
	\fIcurl_multi_add_handle(3)\fP. You can add more easy handles to a multi
	handle at any point, even if other transfers are already running.

	Should you change your mind, the easy handle is again removed from the multi
	stack using \fIcurl_multi_remove_handle(3)\fP. Once removed from the multi
	handle, you can again use other easy interface functions like
	\fIcurl_easy_perform(3)\fP on the handle or whatever you think is
	necessary. You can remove handles at any point in time during transfers.

	Adding the easy handle to the multi handle does not start the transfer.
	Remember that one of the main ideas with this interface is to let your
	application drive. You drive the transfers by invoking
	\fIcurl_multi_perform(3)\fP. libcurl will then transfer data if there is
	anything available to transfer. It'll use the callbacks and everything else
	you have setup in the individual easy handles. It'll transfer data on all
	current transfers in the multi stack that are ready to transfer anything. It
	may be all, it may be none. When there's nothing more to do for now, it
	returns back to the calling application.

	Your application extracts info from libcurl about when it would like to get
	invoked to transfer data or do other work. The most convenient way is to use
	\fIcurl_multi_poll(3)\fP that will help you wait until the application should
	call libcurl again. The older API to accomplish the same thing is
	\fIcurl_multi_fdset(3)\fP that extracts fd_sets from libcurl to use in
	select() or poll() calls in order to get to know when the transfers in the
	multi stack might need attention. Both these APIs allow for your program to
	wait for input on your own private file descriptors at the same time
	\fIcurl_multi_timeout(3)\fP also helps you with providing a suitable timeout
	period for your select() calls.

	\fIcurl_multi_perform(3)\fP stores the number of still running transfers in
	one of its input arguments, and by reading that you can figure out when all
	the transfers in the multi handles are done. 'done' does not mean
	successful. One or more of the transfers may have failed.

	To get information about completed transfers, to figure out success or not and
	similar, \fIcurl_multi_info_read(3)\fP should be called. It can return a
	message about a current or previous transfer. Repeated invokes of the function
	get more messages until the message queue is empty. The information you
	receive there includes an easy handle pointer which you may use to identify
	which easy handle the information regards.

	When a single transfer is completed, the easy handle is still left added to
	the multi stack. You need to first remove the easy handle with
	\fIcurl_multi_remove_handle(3)\fP and then close it with
	\fIcurl_easy_cleanup(3)\fP, or possibly set new options to it and add it again
	with \fIcurl_multi_add_handle(3)\fP to start another transfer.

	When all transfers in the multi stack are done, close the multi handle with
	\fIcurl_multi_cleanup(3)\fP. Be careful and please note that you \fBMUST\fP
	invoke separate \fIcurl_easy_cleanup(3)\fP calls for every single easy handle
	to clean them up properly.

	If you want to re-use an easy handle that was added to the multi handle for
	transfer, you must first remove it from the multi stack and then re-add it
	again (possibly after having altered some options at your own choice).
	.SH "MULTI_SOCKET"
	\fIcurl_multi_socket_action(3)\fP function offers a way for applications to
	not only avoid being forced to use select(), but it also offers a much more
	high-performance API that will make a significant difference for applications
	using large numbers of simultaneous connections.

	\fIcurl_multi_socket_action(3)\fP is then used instead of
	\fIcurl_multi_perform(3)\fP.

	When using this API, you add easy handles to the multi handle just as with the
	normal multi interface. Then you also set two callbacks with the
	\fICURLMOPT_SOCKETFUNCTION(3)\fP and \fICURLMOPT_TIMERFUNCTION(3)\fP options
	to \fIcurl_multi_setopt(3)\fP. They are two callback functions that libcurl
	will call with information about what sockets to wait for, and for what
	activity, and what the current timeout time is - if that expires libcurl
	should be notified.

	The multi_socket API is designed to inform your application about which
	sockets libcurl is currently using and for what activities (read and/or write)
	on those sockets your application is expected to wait for.

	Your application must make sure to receive all sockets informed about in the
	\fICURLMOPT_SOCKETFUNCTION(3)\fP callback and make sure it reacts on the given
	activity on them. When a socket has the given activity, you call
	\fIcurl_multi_socket_action(3)\fP specifying which socket and action there
	are.

	The \fICURLMOPT_TIMERFUNCTION(3)\fP callback is called to set a timeout. When
	that timeout expires, your application should call the
	\fIcurl_multi_socket_action(3)\fP function saying it was due to a timeout.

	This API is typically used with an event-driven underlying functionality (like
	libevent, libev, kqueue, epoll or similar) with which the application
	"subscribes" on socket changes. This allows applications and libcurl to much
	better scale upward and beyond thousands of simultaneous transfers without
	losing performance.

	When you've added your initial set of handles, you call
	\fIcurl_multi_socket_action(3)\fP with CURL_SOCKET_TIMEOUT set in the sockfd
	argument, and you'll get callbacks call that sets you up and you then continue
	to call \fIcurl_multi_socket_action(3)\fP accordingly when you get activity on
	the sockets you've been asked to wait on, or if the timeout timer expires.

	You can poll \fIcurl_multi_info_read(3)\fP to see if any transfer has
	completed, as it then has a message saying so.
	.SH "BLOCKING"
	A few areas in the code are still using blocking code, even when used from the
	multi interface. While we certainly want and intend for these to get fixed in
	the future, you should be aware of the following current restrictions:

	.nf
	- Name resolves unless the c-ares or threaded-resolver backends are used
	- SOCKS proxy handshakes
	- file:// transfers
	- TELNET transfers
	.fi
	.SH "SEE ALSO"
	.BR libcurl-errors "(3), " libcurl-easy "(3), " libcurl "(3) "